Magnetic devices, commonly referred to as bubble domain devices, depend on the use of materials which can maintain and propagate small cylindrical domains or bubbles of reversed magnetization in appropriate magnetic fields. These materials are generally in the form of thin single crystals supported on a non-magnetic, compatible substrate having a matching lattice constant. The devices comprise a film of magnetic bubble material, means for generating localized reversed domains, means for propagating the domains along a predetermined path, such as a conductor circuit, to another portion of the film and means for detecting the presence of the bubbles. The latter is generally accomplished by magneto-resistive techniques which detects the presence of a bubble as an information bit. A high bit density, or small bubble size, for a given film is generally desirable within the limits of conventional photolithiographic fabrication techniques used to make the propagation circuits. Materials research to date has resulted in garnet materials which can form bubbles less then 1 mil in diameter, preferably about 1/4 - 1/3 mil in diameter, so as to obtain a bit density of 10.sup. 5 to 10.sup. 6 bits/square inch of film. However, the bubbles must be greatly expanded, e.g., about 100 - 300 times, in order to detect them by magneto-resistive techniques and this results in a large decrease in the useful area of a particular film on which information can be stored.
Magneto optic techniques have also been tried for detecting bubbles, but in magnetic bubble materials available heretofore, the optical properties, particularly Faraday rotation, are low and high powered lasers or very sensitive light detectors are required to detect the presence or absence of bubbles optically.
Magnetic materials, to be suitable as bubble materials, must satisfy the following general requirements: they must have a uniaxial magnetic anisotropy (K.mu.) with the easy axis of magnetization perpendicular to the film surface; and they must have a saturation magnetization (4.pi.Ms) such that EQU K.mu. &gt; 2.pi.Ms.sup.2.
Owing to their cubic magnetic anisotropy, simple garnets such as YIG are unsuitable for bubble device applications. Liquid phase epitaxial films of doped garnets can acquire uniaxial anisotropy as a result of ionic segregation that occurs during growth. To satisfy the requirements given above, the saturation magnetization of doped garnets is reduced by substituting gallium or aluminum for some of the tetrahedral iron ions. Known garnets are temperature sensitive, such that close control of temperature during operation of devices incorporating them is required.
In addition to the magnetic and optical properties required or desirable for good bubble materials employed in conjunction with magneto-optic bubble detection, the films should also be able to be grown as thin, defect free, single crystal films onto a supporting compatible substrate. Preferably, single crystal films are grown by liquid phase epitaxial techniques from a suitable flux. A lattice constant match within about 0.002 Angstroms is required between the film and that of commercially available substrates, such as gadolinium gallium garnet.